Rapid diagnostic tests for COVID-19, influenza, and blood analytes currently have many drawbacks. They vary greatly in accuracy, are expensive to manufacture, and can be insufficiently sensitive to small antigen loads. Leucine lock is designed to rectify these issues that exist in current rapid diagnostics by creating a rapid antigen test with minimal background activity, tunable sensitivity, and low manufacturing cost. Leucine lock consists of a novel leucine zipper containing an intramolecular hairpin, an ScFv antigen binding domain, and a fluorophore reporter in a FRET combination. By altering the number of hydrophobic and electrostatic mismatches in the intramolecular hairpin, as well as utilizing either a three or four heptad repeat in the hairpin, our results show that the rate of recombination can be controlled. We chose a construct with an appropriate rate of recombination and designed a rapid test for insulin. We show that the test can determine the presence or absence of insulin, and with further optimization can be used to directly quantify insulin levels.